This invention relates to a fuel cell coolant system.
A fuel cell can convert chemical energy to electrical energy by promoting a chemical reaction between two gases.
One type of fuel cell includes a cathode flow field plate, an anode flow field plate, a membrane electrode assembly disposed between the cathode flow field plate and the anode flow field plate, and two gas diffusion layers disposed between the cathode flow field plate and the anode flow field plate. A fuel cell system made up of multiple fuel cells also typically includes one or more coolant flow field plates disposed adjacent the exterior of the anode flow field plates and/or the exterior of the cathode flow field plates.
Each flow field plate has an inlet region, an outlet region, and open-faced channels connecting the inlet region to the outlet region and providing a way for distributing the gases to the membrane electrode assembly.
The membrane electrode assembly usually includes a solid electrolyte (e.g., a proton exchange membrane, commonly abbreviated as a PEM) between a first catalyst and a second catalyst. One gas diffusion layer is between the first catalyst and the anode flow field plate, and the other gas diffusion layer is between the second catalyst and the cathode flow field plate.
During operation of the fuel cell, a reactant gas, e.g., hydrogen, enters the anode flow field plate at the inlet region of the anode flow field plate and flows through the channels of the anode flow field plate toward the outlet region of the anode flow field plate. The other gas, e.g., air, enters the cathode flow field plate at the inlet region of the cathode flow field plate and flows through the channels of the cathode flow field plate toward the cathode flow field plate outlet region.
As the reactant gas flows through the channels of the anode flow field plate, the reactant gas passes through the anode gas diffusion layer and interacts with the anode catalyst. Similarly, as the other gas flows through the channels of the cathode flow field plate, the other gas passes through the cathode gas diffusion layer and interacts with the cathode catalyst.
The anode catalyst interacts with the reactant gas to catalyze the conversion of the reactant gas to reaction intermediates. The reaction intermediates include ions and electrons. The cathode catalyst interacts with the other gas and the reaction intermediates to catalyze the conversion of the other gas to the chemical product of the fuel cell reaction.
The chemical product of the fuel cell reaction flows through a gas diffusion layer to the channels of a flow field plate (e.g., the cathode flow field plate). The chemical product then flows along the channels of the flow field plate toward the outlet region of the flow field plate.
The electrolyte provides a barrier to the flow of the electrons and gases from one side of the membrane electrode assembly to the other side of the membrane electrode assembly. However, the electrolyte allows ionic reaction intermediates to flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly.
Therefore, the ionic reaction intermediates can flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly without exiting the fuel cell. In contrast, the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly by electrically connecting an external load between the anode flow field plate and the cathode flow field plate. The external load allows the electrons to flow from the anode side of the membrane electrode assembly, through the anode flow field plate, through the load and to the cathode flow field plate.
Electrons are formed at the anode side of the membrane electrode assembly, indicating that the reactant gas undergoes oxidation during the fuel cell reaction. Electrons are consumed at the cathode side of the membrane electrode assembly, indicating that the other gas undergoes reduction during the fuel cell reaction.
For example, when hydrogen and oxygen are the two gases that are used in the fuel cell, the hydrogen flows through the anode flow field plate and undergoes oxidation. The oxygen flows through the cathode flow field plate and undergoes reduction. The specific reactions that occur in the fuel cell are represented in the following Eqs. 1-3:
H2xe2x86x922H++2e31xe2x80x83xe2x80x83(1)
xc2xdO2+2H++2exe2x88x92xe2x86x92H2Oxe2x80x83xe2x80x83(2)
H2+xc2xdO2xe2x86x92H2Oxe2x80x83xe2x80x83(3)
As shown in Eq. 1, the hydrogen forms protons (H+) and electrons (exe2x88x92). The protons flow through the electrolyte to the cathode side of the membrane electrode assembly, and the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly through the external load. As shown in Eq. 2, the electrons and protons react with the oxygen to form water. Eq. 3 shows the overall fuel cell reaction.
In addition to forming chemical products, the fuel cell reaction produces heat. One or more coolant flow field plates are typically used to conduct the heat away from the fuel cell and maintain appropriate stack temperatures.
Each coolant flow field plate has an inlet region, an outlet region, and channels that provide fluid communication between the coolant flow field plate inlet region and the coolant flow field plate outlet region. A coolant, e.g., liquid de-ionized water or other low conductivity fluids, at a relatively low temperature enters the coolant flow field plate at the inlet region, flows through the channels of the coolant flow field plate toward the outlet region of the coolant flow field plate, and exits the coolant flow field plate at the outlet region of the coolant flow field plate. As the coolant flows through the channels of the coolant flow field plate, the coolant absorbs heat formed in the fuel cell. When the coolant exits the coolant flow field plate, the heat absorbed by the coolant is removed from the fuel cell.
To increase the electrical energy available, a plurality of fuel cells can be arranged in series to form a fuel cell stack. In a fuel cell stack, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include monopolar plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells.
In one aspect of the invention, a fuel cell system includes a fuel cell stack including a plurality of fuel cells, a coolant inlet manifold, and a coolant outlet manifold. A coolant expansion tank has an input connected to the coolant inlet manifold of the fuel cell stack and an output connected to the coolant outlet manifold of the fuel cell stack.
In another aspect of the invention, a coolant expansion system includes a coolant expansion tank configured to accept coolant at an input connected to an inlet coolant manifold of a fuel cell stack, to allow gas included in the coolant to escape into an open air space above the coolant in the coolant au expansion tank, and to release coolant through an output connected to an outlet coolant manifold of the fuel cell stack.
One or more of the following advantages may be provided by one or more aspects of the invention.
Providing a coolant expansion tank allows coolant flowing through the fuel cell stack to expand into the coolant expansion tank and maintain coolant pressures within the fuel cell stack to within acceptable levels. Gases trapped in the coolant can escape into the coolant expansion tank, thereby innocuously localizing these gases in the coolant expansion tank rather than in the fuel cell stack and allowing coolant with improved heat-absorbing capacity to circulate back through the fuel cell stack. In addition, the coolant level in the fuel cell system can be monitored by observing the amount of coolant in the coolant expansion tank.
Other features and advantages of the invention will be apparent from the detailed description and the drawings.